1. Field of the Invention
The present invention relates to measuring heads of an atomic force microscope AFM, and particularly to a measuring head of an atomic force microscope by the light beam lever system.
2. Description of the Background Art
A measuring head of an atomic force microscope includes as its basic components a light source for generating a light beam, a cantilever for reflecting the light beam and a light position detector for detecting "deflection" (displacement) of the cantilever according to positional change of the light beam reflected by the cantilever. References shown below disclose structures of such a measuring head of an atomic force microscope by the light beam lever system.
(1) Gehard Meyer and Nabil H. Amer: Appl. Phys. Lett., Vol. 53, No. 12 (1988), pp. 1045-1047.
(2) B. Brake, C. B. Prater, A. L. Weisenhorn, S. A. C. Gould, T. R. Albrecht, C. F. Quate, D. S. Cannell, H. G. Hansma, and P. K. Hansma: Science, Vol. 243 (1989), pp. 1586-1589.
(3) S. A. Chalmers, A. C. Gossard, A. L. Weisenhoru, S. A. C. Gould, B. Drake, and P. K. Hansma: Appl. Phys. Lett., Vol. 55, No. 24 (1989), pp. 2491-2493.
(4) Gehard Meyer and Nabil M. Amer: Appl. Phys. Lett., Vol. 56, No. 21 (1990), pp. 2100-2101.
(5) R. C. Barrett and C. F. Quate: J. Vac. Sci. Technol. A, Vol. 8, No. 1 (1990), pp. 400-402.
(6) Gabi NeuBauer, M. Lawrence A. Dass, and Thad Jhonson: Proc. of IEEE Intern. Rel. Phys. Sympo., (1992) pp. 299-303.
(7) M. Radmacher, R. W. Tillmnann, M. Fritz, H. E. Gaub: Science, Vol. 257 (1992), pp. 1900-1905.
(8) P. K. Hansma, B. Drake, D. Grigg, C. B. Prater, F. Yashar, G. Gurley, V. Elings, S. Feinstein, and R. Lal: J. Appl. Phys., Vol. 76, No. 2 (1994), pp. 796-799.
(9) U. Rabe and W. Armold: Appl. Phys. Lett., Vol. 64, No. 12 (1994), pp. 1493-1495.
(10) Japanese Patent Laying-Open No. 2-281103.
(11) Japanese Patent Laying-Open No. 4-285810.
(12) Japanese Patent Laying-Open No. 7-5181.
FIG. 12 shows a structure diagram of a conventional measuring head of an atomic force microscope disclosed in each of the above references (1) through (8). FIG. 13 shows one disclosed in the reference (9), which shows a structure of a conventional measuring head to which a mirror 51P for changing the optical path direction is added. FIG. 12 shows a control portion 10P for operating the measuring head 100P, too, which shows the entire system as an atomic force microscope 20P.
A light beam 41 emitted from a light source 4 is converged onto the upper surface of a cantilever body 1, and a reflected light 42 thereof is incident upon a light position detector 5. This light position detector 5 includes a two-segment or four-segment photodiode, which detects positional shift of the reflected light 42 from the cantilever body 1 to sense fine "deflection" of the cantilever body 1 caused by the atomic force occurred between a probe 2 provided at the end of the cantilever body 1 and a measured sample 3. The light source 4 emits a light beam 41 which is not linearly polarized, the cantilever body 1 is composed of a material such as silicon (Si) or silicon nitride (Si.sub.3 N.sub.4), etc., the probe 2 is composed of silicon (Si) or diamond, etc. having a sharp tip, and the light position detector 5 simply detects the positional change of the incident light.
The operation for measuring an image of unevenness on the surface of the measured sample 3 by using this atomic force microscope 20P will be described below.
First, a controller 7P of the control portion 10P receives an instruction from a computer 8P and applies a voltage to a Z electrode of a cylindrical piezo element 6 to move the measured sample 3 in the Z direction (up and down direction) with feedback control so that the reflected light 42 from the cantilever body 1 is incident on a certain position on the light position detector 5, i.e., on a position displaced by a certain amount from a position where an output of the light position detector 5 attains just 0. Then, this way, while the cylindrical piezo element 6 is operated with feedback in the Z direction, a voltage is further applied to an XY electrode of the piezo element 6 from the computer 8P through the controller 7P to simultaneously scan the measured sample 3 in the XY directions, too. At this time, by reading each voltage in the XYZ directions applied to the cylindrical piezo element 6 from the controller 7P, an image of the unevenness on the surface of the measured sample 3 is obtained.
However, the conventional measuring head disclosed in the references cited above generally have such problems as below.
Now, FIG. 14 and FIG. 15 are a side view of the measuring head 100P and a transverse cross section seen from the incident side of the light beam 41 illuminating the vicinity of the end portion of the cantilever body 1, respectively, which are shown to illustrate problems of the conventional measuring head of the atomic force microscope.
First, when the conventional measuring head 100P is applied to the measured sample 3 having a considerably uneven surface 3a as shown in FIG. 14, the measurement accuracy in the Z direction (up and down direction) is considerably decreased. This is described in detail below.
Generally, in the atomic force microscope, the thickness and the width (w) of the cantilever body 1 (refer to FIG. 15) are set small so that even fine atomic force can cause large "deflection" of the cantilever body, accordingly, to reduce a spring constant thereof. On the other hand, the light beam 41 having a sectional dimension or a beam diameter (D) larger than the width (w) of the cantilever body 1 is used so that a large output signal is obtained in the light position detector 5, and further, so that the light beam 41 can be certainly incident on the cantilever body 1 and the illumination thereof can be visually recognized. Accordingly, as schematically shown in FIG. 15, the sectional dimension (D) of the light beam 41 is larger than the dimension of the cantilever body 1, e.g., then the width (w), and then the incident light beam 41 includes not only a necessary light 41a actually required to obtain measured values in the Z direction but also an unnecessary light off the frame of the end portion, i.e., an extra light 41b which will cause measurement errors. Accordingly, not only a regularly reflected light 42a originated from the necessary light 41a, but also an irregularly reflected light 42b produced by irregular reflection of the extra light 41b off the cantilever body 1 on the surface 3a of the measured sample 3 is incident upon the light position detector 5, depending on the interval between the cantilever body 1 and the surface 3a of the measured sample 3 and the unevenness on the surface 3a, resulting in a decrease of the measurement accuracy in the Z direction mentioned above.
As a result, correct measurement in the Z direction with the regularly reflected light 42a can not be made, causing the problem of confusing the feedback control mentioned above. Especially, when one of the regularly reflected light 42a and the irregularly reflected light 42b causes the output of the light position detector 5 on the plus side and the other causes the output on the minus side, the output fluctuating according to the intensity of light all the time may disable the control. This way, in the conventional measuring head 100P, the light beam 41 is reflected also on the surface 3a of the sample and the irregularly reflected light 42b not from the cantilever body 1 strays into the light position detector 5. Accordingly, for example, the atomic force microscope using the conventional measuring head 100P could not accurately observe and measure the surface of the measured sample 3 having fine, complicated and extremely uneven pattern configuration, such as a semiconductor chip surface etc. The problem of the conventional measuring head 100P has arisen because the irregularly reflected light resulted from the extra light 41b was not recognized and no measures have been taken thereto, as mentioned above, which will be exemplified by the force curve shown in FIG. 5 later.
Now, it may be supposed that the above-described problem can be solved by converging the light beam from the light source with a lens to narrow down the beam. Considering it from the user side, however, it is difficult to satisfy the resolution above, and which will be an impractical method. In the atomic force microscope, a structure is desired in which the beam diameter of the light beam is large enough so that it can be visually recognized that the light beam is certainly radiated onto the end of the cantilever body. Under such a realistic demand, a measure is earnestly required to solve the problem of the irregular reflection light.